1. Field of the Invention
The invention relates generally to the field of energy storage. More specifically, the invention relates to an energy storage apparatus for storing electrical energy.
2. Description of the Related Art
Recently, ultracapacitors have become popular for the storage of electrical energy, because ultracapacitors can provide high levels of capacitance, e.g., 3,000 farads (“F”), with high output current, e.g., 500 Amperes. These high levels of capacitance are facilitated by the small spacing between the electrodes of the ultracapacitor, which can be less than a few nanometers.
When compared with electrochemical storage devices, i.e., batteries, ultracapacitors offer significant advantages. For example, ultracapacitors can provide ten to thirty times the cycle life of batteries. Also, ultracapacitors require less maintenance than batteries, are more efficient than batteries, and are easier to manufacture, and therefore, cheaper to produce, than batteries. In addition, ultracapacitors are more environmentally friendly to manufacture and to dispose of than batteries, because ultracapacitors lack many of the environmentally damaging materials, e.g., lead, nickel, cadmium, and mercury, that are included in batteries.
Furthermore, ultracapacitors can be configured in parallel, as well as, in series, thus, providing more energy storage capacity and high current capability. On the other hand, batteries are more difficult to configure in parallel, especially at the module level, due to circulating currents that are created between battery modules that are connected in parallel. Also, it is difficult to provide balanced charging to the battery modules that are connected in parallel.
Despite these advantages, ultracapacitors are still inferior to batteries in terms of their specific energy and energy density. For example, when compared to lead-acid batteries, a widely used battery chemistry with a typical specific energy of 20 Watt-hour/kilogram (“Wh/kg”), the specific energy of ultracapacitors are still about one fourth that of lead-acid batteries, i.e., approximately 5 Wh/kg. Also, the voltages for ultracapacitors have remained low, typically about 2.5 V.
The energy stored in an ultracapacitor is given in the following equation:E=½C*V2 where:                E=energy in Watt-hours,        C=capacitance of the ultracapacitor in farads, and        V=the voltage across the terminals of the ultracapacitor in volts.By doubling the voltage across the ultracapacitor's terminals, the energy stored in the ultracapacitor is quadrupled. This increase in the value of the voltage across the ultracapacitor's terminals will lead to a specific energy of approximately 20 Wh/kg, assuming that the subsequent increase in the mass of the resulting ultracapacitor is small. Such improvements will bring the specific energy of the ultracapacitors very close to that of typically lead-acid batteries.        
A clear limitation associated with ultracapacitors is that as the energy stored in the ultracapacitor is used, the voltage across the ultracapacitor's terminals decreases in value. This is evident from the above equation, and the curve depicted in FIG. 1, which shows the decrease in the value of the voltage across an ultracapacitor's electrodes as a function of the amount of energy delivered by the ultracapacitor. For instance, when 75% of the energy stored in the ultracapacitor has been used, the voltage across the ultracapacitor decreases to half its initial value. Since a significant portion of the electrical energy stored in an ultracapacitor must be used in order to achieve a high specific energy, the voltage across the terminals of an ultracapacitor must substantially decrease in value over time. For most applications, such a fluctuation in voltage is significant and undesirable. It should, therefore, be appreciated that there is a need for an efficient device that maintains the voltage across an ultracapacitor while the electrical energy is output from the ultracapacitors. The present invention satisfies this need.